bug#33302: Unexpected unzip

[d . s . robertson]

I am using raspbian stretch, terminal window. superuser activated.
In directory /var/log/apt the history.log.1.zg file is marked as gz format.
I want to look for lines with the word install in them.
I didn't necessarily want to uncompress the file. I executed the command
gunzip history.log.1.gz | grep install
My expectation was that the un zipped contents would be piped to grep which 
would filter for lines containing the word install.
It changed the file to a decompressed version. I don't think it changed the 
contents. There was no terminal output.
I gather I have to use -c
I just wonder if a pipe might automatically be interpreted as providing the 
contents and leaving the original unchanged?

David

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